Use a Palm! Or a souped-up calculator. Or something along those lines.

I've been playing Donjon for a few weeks now; most of that has been online, over IRC (hopefully I'll have the logs posted at some point so people can see how Donjon games play out). We use a fairly basic dice-bot for our rolling needs. It's pretty easy. Someone types a command like:
5 d20
And the bot'll return:
11 9 1 15 4

..or something along those lines. You can do things like '5 5d20' (so it would roll 5d20 five times, and display each result), or 5 d20+4 (each roll would get 4 added to it), and so on. Works great.

And then my sibling and I start up a table campaign; everyone's mentioned their first forays into Donjon and their lack of d20s, so that hardly needs repeating here. ;) The dice-rolling was actually the minor part; it was comparing the rolls that got a bit tedious in a game that's supposed to be pretty fluid.

So, we invested in an old-but-working monochrome Palm Pilot-thing for around $40. Saves a great deal of time. The downside is that it's not nearly as fun pressing '10 d20' as it is actually rolling those ten dice. The upsides are that it's faster, easier to compare player/GM tests, less bulky, and you're not restricted to using real-world die sizes for your game if you so choose ("I want to roll 37-sided dice!")

Anyway, it's something to think about. If it it's only a few dice (1-5) we usually use real ones, but it's a godsend for the occasional 15+ rolls we have.

edit: A laptop would also work pretty great as well.

I've done this in the past. What I'd really like to see is a dice rolling program that would also sort the dice for you. Right now, I prefer the physical dice because I can arrange them in order for faster reference. Do you have any recommendations?