Being a geek in my circle of friends, I am always asked “What should I install on my phone?” Since I just got a Nexus One recently, let me share with you 18 of the essential apps that you should install in your Android phone.

1. WaveSecure

In my opinion, WaveSecure is the first app you should install upon getting your phone. WaveSecure is a mobile security software that locks your phone when it is stolen (or lost). When you first run the app, it will register your SIM card ID and your phone IMEI number in its database. When it detects that the SIM in the phone is different, it will auto-lock the phone and can only be reactivated with your own pin. It will also send a notification sms to your buddy as well as an email to you. Optionally, you can get it to wipe your phone data over the air.

2. TasKiller

There are several task killing applications in the market, and their job is to kill the background tasks to prevent them from taking up extra memory and battery life. Personally, I recommend TasKiller because it has the AutoKill feature that kills all applications when you switch off the screen. This saves you the effort to open the app and kill the tasks manually. It also comes with an IGNORE list so you can define which apps should not be killed.

3. Astro File Manager

A file manager is almost a must in all computers and smartphones (surprise that iPhone doesn’t come with one). In Android, there is no lack of file manager apps in the market, but the one I used religiously is Astro File Manager. It is simple to use and allows you to backup/restore your apps to/from the SD card. You can also open media files directly from the app. Some other alternative file managers you can consider include Linda File Manager and EStrongs File Explorer.

4. Battery widget

Somehow, Android does not provide you with an easy way to monitor your battery status. The battery indicator at the top does not show how much battery life your phone has and is a total waste of screen real estate. Battery widget is a widget that sits on your screen and does nothing but to display the percentage of battery life. Simple, but get the job done. Note: There are two batter widget apps in the market, one is developed by Mippin.com while the other is developed by HTC. The one I am referring to is the one developed by Mippin.com. HTC users should however get the one developed by HTC.

5. APNdroid

Turning your 3G/GPRS network off when not in use is a good way to conserve battery life. However, when you can’t find the option to turn it off (some build of Android, especially Nexus One, don’t come with the option to turn it off), APNdroid will be the one you should look for. APNdroid modifies the APN names and prevents your phone from connecting to internet over 3G/EDGE/GPRS.

6. Handcent SMS

There is really nothing to complain about the default messaging app, it’s just that Handcent SMS makes messaging much easier and enjoyable. The feature I like most is the popup screen when a new message arrives. You can instantly reply on the popup and it saves you the effort to open up the messaging app. In addition, there are various conversation style to choose from and tons of customization settings for the control freaks to play with.

7. SlideME Marketplace

While the Google marketplace grants you access to free apps, the paid apps are only available in certain countries. In order to access the paid apps without having to root your phone, the best way is to install SlideME Marketplace. The SlideMe is not available in the Android market, you have to browse to the SlideME site and install the app manually. One thing though, all the apps in this marketplace are unmoderated. While there are several gems, there are also plenty of spams and craps. So be more vigilant when installing apps.

8. Facebook

The official Facebook app is awesome. It’s almost an exact clone of the iPhone Facebook app. You can scan your news feed, view your friends’ walls and user information, comment on status updates, hit the “like” button, take and upload photos, add new friends and post status updates of your own. Nexus one comes with Facebook preinstalled and it is deeply integrated with your Google contacts.

9. Astrid Task

Astrid is one of the best (if not, the best) ToDo list app in the whole Android market. It sync with RememberTheMilk and allows you to set alarm, tags, due date for your tasks. It even comes with a widget that shows your to do list on the screen. Cool!

10. GDocs

Google has integrated plenty of Google Apps in the Android platform, but one thing that is missing is the Google document. GDocs covers this gap, though not perfect, but it gets the job done. You can view, edit and sync documents from your Google Docs account. It does not support spreadsheet at the moment. Any attempt to open the Google spreadsheet in GDocs will direct you to the Google Docs mobile site.

11. SMS Backup & Restore

As its name implies, it backups and restore your SMS to your SD card. You never know when your phone is going to crash, so it is best to have a backup in hand.

12. Seeesmic

Previously, Twitroid is my favorite Twitter client, but ever since Seesmic releases a new version that support multiple twitter accounts, it has become my favorite. The best about Seesmic is the grouping of all your Twitter timelines in one simple screen. You can just scroll infinitely to read your tweets and not be bothered about switching screens.

12. Bluetooth File Transfer

Use your Android phone to browse, explore and manage files of any Bluetooth ready device. It is good for transferring files between your phone and PC when you don’t have the sync cable around.

13. PdaNet

Tether your Android phone without having to root the phone. You can be connected to either 3G, WiFi, or even through VPN on your phone and PdaNet will share the connection with your computer. The free version blocks secure sites, other than that, it works perfectly fine.

14. ReaderScope/ NewsRob

If you are just looking for a pure RSS reader, then I would strongly recommend NewsRob. However if you require more stuffs like istening to Podcasts. Watch YouTube videos. Share over SMS, Email, Twitter, Del.icio.us. Check the buzz on Digg, Reddit, Friendfeed etc, then ReaderScope is the one for you. Both apps support Google Reader and give you option to cache the feed for offline use. NewsRob is more polished and can cache up to 1000 feeds. ReaderScope has more features, but can only cache up to 100 feeds.

15. Dolphin browser

If you are looking for multi-touch (except for Nexus One) and tabs function in your mobile browser, then Dolphin browser is a good choice. Dolphin browser comes with tons of great features including gestures and bookmarks sync, but the performance can be a bit slow and lagging occasionally.

16. Fring

Connect to various IM networks and make phonecalls over Skype and SIP. There have been some issues with the previous version, but the latest update has fixed lots of bugs. It is now working fine on my Nexus One.

17. Mabilo Ringtone/Ringdroid

These two apps both deal with ringtone. Mabilo Ringtone allows you to download free (and nice) ringtones from their server while Ringdroid allows you to create your own ringtone. Personally I don’t like the ringtones that come with the phone, so these two apps have been very useful for me to customize my ringtone.

18. Evernote

It’s hard not to love Evernote, especially when they added the offline support to their app. You can now record anything either with a note, a snapshot or even a voice recording and get it sync instantly to the Web account. There is no excuse for you to forget anything again. What other apps have I missed out?