Helix Launcher (1MB) - After installing about 4-5 different 'Home' replacement apps I have finally settled down for the HelixLauncher. It has a nice light design and is very smooth. The best thing I like about this are the quick launch buttons which can be customised and the fact that double tapping gives you a preview of the screens you are running.
Advanced Task Manager (FREE) (356KB) - Simply the best out there, the "Press to End" widget is god sent especially on a phone like Samsung Galaxy with its meager 128 MB RAM. After install ensure the required apps are put in the "Ignore" list and then just use the "Press to end" widget shortcut to free up memory whenever you feel the phone is sluggish. Works great.
Handcent SMS (2MB) - Although very similar to ChompSMS, I think it outshines the ChompSMS by a slight margin because of the some extra goodies. Definitely a 5 star application.
Battery Life (115KB) - Beautiful widget which changes color according to the battery level. A must have for any android phone.
ThickButtons (BETA) (911KB) - A pretty accurate keyboard which predicts the next key you will press and enlarges those keys. Can be a pain when typing non standard words though.
Swype (BETA) (2 MB) - A unique keyboard. Definitely worth a try. Although my beta is working in reduced support mode after I reflashed :-(.
SMS Backup (661KB) - A nifty little utility which will backup your text messages to your gmail account ! Needs IMAP enabled for gmail though.
3Banana (548 KB) - Brilliant Note taking app which syncs with an online snaptic account. You can use your google account to sign in.
Color Flashlight (221 KB) - A simple yet very useful app. Love the various effects you can use. :-)
APN Backup & Restore ( 143 KB) - Does what it says.
Other Apps :
twicca BETA (1 MB) - One of the best UI of all the apps I tried for twitter. A gorgeous widget to go with it.
Bloo (581 KB) - for all the facebook fans out there. Although I haven't compared it much to the other apps, I find it better on the face of it.
Colordict (820 KB) - A unique implementation of dictionary, thesaurus, and Wikipedia all rolled into one app. Best part is that after the installation it copies the data to the SDCard after which you can uninstall the addons.
Others worth a mention : Barcode Scanner, Metal Detector, Better Terminal Emulator, Unit Convertor, Gesture Search. Astro FileManager
The one thing that influenced my selection of apps is the ability to backup the data in the cloud. That way I don't have to worry about losing my phone or taking a backup before I flash and reflash again. The other most important thing is the size of the app.
Hope this helps, please share your favorite apps in the comments.