Ah, one of my favorite topics. I wrote DM's Familiar (DMF) so I'm a little biased. I'll make general comments instead of specific. And you asked about "at the gaming table" so I won't talk about prepping for the game. Things to use at the gaming table:
Text Organizer: Something to organize your adventure, campaign info, player info, etc. etc. etc. Word being the easiest answer. MyInfo is kind of nice. DMF has a "Codex Tree" which is an outline/tree/text tool similar to MyInfo.
Reference Material: A PDF/HTML of the SRD is nice for looking up info but what about all your supplemental books? A program that you can easily get info into and out of is key. Needs an easy and quick way to look up spells, feats, rules, etc. You shouldn't have to carry any books with you to the gaming session.
Dice roller: An easy way to roll the dragon's 17d6 fireball and add it up - that's the basic. Would be nice if the program new all of your player's Listen checks and could secretly make that Listen check for you without tipping them off (DMF does).
Initiative Tracker: A handy program just to keep track of who's turn it is, when a spell duration has ended, etc.
Combat Manager: Make attack and damage rolls for you to keep the game moving quickly.
The DM Assistant programs out there do most of these things in one shape or another: DM's Familiar, DM Genie, Roleplaying Master, Campaign Suite.
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Other than DM's Familiar, I use a couple specific programs:
Tablesmith : A program for generating random results of any type. I've got a ton of tables in it for random NPC names and traits, the menu at the inn the PCs stop at, description of gems, name of a shop, etc.
Music: Some type of music player for background music and sound effects.