ServerInfo

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This nifty little program allows you to monitor a server or servers; auto-connect when a public slot is free, a player connects, etc; alert you when a player connects; and search WON like GameSpy. I like this program because it is much cleaner and uncluttered than all the other ones I've seen out there. :)
Refunct the Defunct

40Kb, 180 downloads
This will fix your function keys. (F1 and F2)
Simple-Script for Natural Selection

115Kb, 573 downloads
This is a script pack that will simplify your gaming experience. It contains one-touch radio commands for Marines and drop/build ones for the Commander. These will over-write your "F" keys, and spacebar will be crouch-jump. I really haven't found much use for scripts with the Alien side other than the crouch-jump that enables you to blink by jump-clicking, so there are none.
Half-Life IP Launcher

354Kb, 248 downloads
This neat little program allows you to connect to a server using its IP number. Just select the MOD and punch in the IP number. It works with passwords as well.
Pak Explorer

170Kb, 487 downloads
Explore your Halflife .pak files with this neat little program. Pak filfes contain sounds, models, sprites and stuff. They are in your half-life/valve folder. (pak0.pak, pak1.pak)
UO Trace

39Kb, 184 downloads
A nice traceroute utility originally used for Ultima Online servers, but can be used for anything...like NS servers. :)