Well for years we used mini's and a hand-drawn map on a dry-erase mat. That was great for 2 reasons : Minis can be fun., and it was pretty quick if you had to improvise. However, perversely enough, it was quite hard to get the dimensions right when I was copying an existing map to the table.
Also, as soon as the GM went for the markers people would know there was a combat coming up

Over the last few months, we've moved to Maptools.
The advantages of MapTools are many : We can jump to a new encounter nearly instantly, provided I've made the map ahead of time. And if you've got a stock of pre-drawn maps from, say, Wizards' Dungeon-a-Week, it's not too hard to come up with a map pretty quickly, even if you weren't prepared for it. The drawing tools are decent as well, even for quick stuff.
Also, I have some friends who live quite a distance away. Now they can join in over the internet much more easily than before.
Maptools also supports a fairly powerful macro toolkit - it has it's issues, but there are a lot of people who write frameworks that run inside Maptools to help automate the GM's job. It's pretty cool.

The downside? I bought a lot of mini's and I don't get to use them any more...