Second hand SB600. Or SB800 as its much more powerfull and probably not much more 2nd hand.
I have one of each.
The new SB900 is lovely and easier to use, but the “old” flashes are fine.
To really get the best out of them you need to get them off camera and use wireless TTL, not sure if the D5000 can act as a flash commander (my ageing D80 can). The SB800 and SB900 can act as master/control flashes, but this only matters when you have a second flash!
I tend to use both my SB600 and SB800 wirelessley and use the cameras onboard flash to control them.