Tools
Perun said:
programmes that can aid in DMing.
That covers a wide-range of things. Any more specific?
First off, you have Character Generators - programs for making NPCs, PCs, advancing monsters, applying templates, etc. In this category is E:Tools, PCGen, Campaign Suite, Roleplaying Master, DM Genie.
Then there's Combat Management programs - programs to use at the game table to run the game. For me, this is the biggest leap in productivity for a DM. In this category is my program,
DM's Familiar, Roleplaying Master and DM Genie.
Then we have Reference programs - programs designed to let you look up rules and other information quickly while you play. DM's Familiiar does that, there's a number of Excel spreadsheets floating around, or the HTML and PDF versions of the SRD.
Lots of DMs seem to like mapping programs - programs for making maps, and I assume, using them during the game. The new Dunjinni, Campaign Cartographer, AutoRealm, and others.
Then there's Miscellaneous. Tablesmith for creating random table of all sorts, character portrait creators, programs for online gaming, etc.
The key point would be that almost all of the programs have free demos. Download the demos and see which ones meet your needs.