Overclock, theme, flash ROMs for better features, speed, etc.
You know, basically anything your phone is capable of. Congrats on taking the training wheels off.
Let me wobble a bit...
I have no idea how to do all the ROM flashing and such and the millions of pages are a bit daunting. A "dummies guide to getting the most out of a rooted droid" would be a God send.
Okay, let me add steps one, two, and three.
1: Get ClockworkMOD from the market, flash in Clockwork Recovery or SPRecovery (not required if you rooted manually and already have SPRecovery), or use the popular RSDLite (access to a Windows PC required) to flash SPRecovery onto your phone.
2: Make a nandroid backup. At some point you'll want to copy this to your PC for emergency purposes.
3: Decide what you want from your now-rooted Droid.
3a:If it's themes, check out Black Glass by 928Droid as a first stop. It's potentially the most "themed" theme of them all. (Opinion, please do not flame). It's a good starting point in part because it's so complete for so many ROMs, including stock.
3b:If it's overclocking, for the love of all that's holy, unholy, profane, sacrosanct, defiled, reviled, beloved, consecrated, cursed, blessed, and peanuts, be careful! Every kernel is different, every Droid is different, and every experience will be different. Taken from another post I made earlier:
For example, P3Droid has kernels that clock as low as 125Mhz and otherwise-identical kernels that only go down to 250Mhz. The reason for this is poor performance under 2.1 that was fixed for 2.2. Pick whichever ones you want based on that.
ChevyNo1 has Ultra-Low-Voltage, Low-Voltage, and Standard-Voltage kernels with a plethora of features built in. Ultra-Low-Voltage kernels use less juice to push your phone's CPU faster, ultimately saving you some of the battery life you're sacrificing by running it so fast. Not all phones can handle the ULV or even the LV kernels common to overclocking on the Droid, and some phones will run very poorly on some kernels. The trick is experimenting.
There are so many kernels out there you should be able to find one that works with your phone... But there's no guarantee
your phone will be able to go that fast.
The most common problem faced by overclockers is stability or the dreaded bootloop. If you get locked in a bootloop pull the battery, put it back in, hold down X on the keyboard, then power the phone back up so you can boot into recovery and reinstall the nandroid backup you made prior to installing the new kernel.
You did remember to make a nandroid backup before you installed the new kernel, didn't you?
3c: Tethering. If you are at all interested in tethering, especially WiFi tethering, rooting is the only way to get it on the Droid. Go to the market and download one of the two apps (or pay for a donation version) and test it, or go straight to the horse's mouth and download the .apk from
android-wifi-tether - Project Hosting on Google Code and give it a whirl. If you EVER plan to USB tether, either use the included option (which may redirect you to a Verizon page asking for money), or download the .apk from the link in my signature.
There's a lot more, like changing options you can only get in certain ROMs, Titanium Backup, the terminal emulator, etc, but these are the three basic and probably most-used root-only functions.
If you have any questions, please ask, as I didn't do a step-by-step, just a general guideline.
PLEASE MAKE NANDROID BACKUPS BEFORE FLASHING ANYTHING IN!