I will give you some things to search in the forums, and I hope your "new" N900 has a good system board and CPU, mine lasted less than a week.
Cool stuff to install:
CSSU even geekier CSSU-thumb
Kernel for power users
Overclocking is tricky especially on an old refurb CPU, use with caution
Sygic mobile maps, works offline
Preenv for over 60 top shelf WebOS games
USBmode for host mode, USB female cables are cheap on eBay, easily use USB gadgets and thumb drives with the N900
flopswap, move your swap wear from the internal flash memory to a SD card
swappolube, stock swap settings suck, fix them!
the Flash 10 and faked 12 hack
720p video driver upgrade
FMradio and FM transmitter boost
Accelymote &mupen 64 gives you accelerometer controlled Mario64 and other N64 games
catoriseplus to group your apps by type
Pyobd and a bluetooth OBD plug to diagnose your car problems
almostTI85, emulate your college TI scientific calculators
mobilehotspot share your 3G internet with your laptop via wifi
rootsh Debian, install debian apps from debian repos
evopedia, download prepared wikimedia dumps for test of all of wikipedia, wikivoyage, and wiktionary 100% offline
mstardict, good for foreign language translation
cutetube, watch youtube and convert youtube music vids to mp3
qtrapids is the best torrent app, look on forum for how you can convert magnet links to torrent files for compatibility
backupmenu, boot time backup tool, make and save backups access your flash drives via USB cable, etc...save your ***!

cool hardware hacks
Double scud battery/mugen cover
Palm touchstone inductive charging, no cable charging means less chance of breaking the USB, save that for host mode or transferring files
Look for my BH-214 28hours of music hacked multimedia bluetooth headset
When you charge and output audio wirelessly get some dust plugs for the USB and audio jack, also protects against some splashes and rain.

just get back with the quality of N900 once u received.. I am planning to buy one from him..

Okay, so I recieved my phone today. Like a few minutes ago. I open the box, put in the sim, but in the battery, connect the wall charger and the phone does this shaky thing with a dim-lighted nokia logo on the screen. Does this for a while but it wont boot, now it shows a constant orange/yellow'ish LED with a dim nokia logo but still can't get it to boot, I think it's just because I need to charge it, not sure though. I will get back with more info later.

So an update again for visN900. As said earlier I have received the phone but it wont boot. I just ordered a new battery and we'll see if that'll fix it. If not I will be sure to let you know. If it works I will do a little testing to find out if the OS is genuine or not and also let you know.

tks for your reply.. any damages with the mobile?? is everything looking good as new??

Sounds like a dead or empty charged batter... Those cheap clone batteries are worthless.

I was in the same situation, it was a problem with the usb. No need to change just to press it

I think it's a battery thing too. I ordered a new battery so I hope it'll fix it. The phone has no scratches and actually seems brand new. The only thing that worries me is the box had been opened, though all packaging inside was undamaged/unopened. He just put in a letter telling me to rate him 5 stars on ebay and if I experience any problems I should contact him before rating so he can help me fix it.

Anyways, the new battery should also have more battery life than the old and I was planning to replace it for that reason too, so if it works with that Im happy. Didnt waste any extra money I wouldnt have spent anyway.

EDIT: Also if you are interrested in shipping I ordered on 8th of aug and received it the 14th. Seems pretty fast to me. I also live in EU (specific. Denmark) and on his own initiative - I did not ask him for this - he marked the shipment as a gift and valued it at $15 so I avoided customs and everything. This is pretty nice I think, since the only reason for him to do that is to be nice to me. If he got insurance he would only be reimbursed $15 if the goods got damaged during the shipping.

what price you paid

I paid 145 us dollars as listed on the eBay link posted earlier. Oh somehow it has dropped to 140 us dollars. ^^ Unlucky me.