Ideas:

1. Take it to a bike shop and ask. They might have the right tools
2. An angle grinder
3. Hacksaw (electric preferably. Maybe an extra blade)
4. Big bolt cutters
5. a small car jack. Most u-locks lock on one side and connect with an L on the other, so bending the U open will release it
6. could you close the lock without the key? Cut away any rubber and slide a shim made from an aluminum can down to release the latch

Ideas:

0. If you still have any of the original paperwork or packaging, look for a lock code/number, you might be able to order replacement keys.
1. Take it to a bike shop and ask. They might have the right tools. They likely do this every once in a while
2. A machine shop is sure to have tools capable of doing this
3. The local police department will have the tools or be able to refer you to somebody that does (they have to deal with removing abandoned bikes)
4. An angle grinder, or other rotary cutting tool with a metal-cutting disk
5. Hacksaw (electric preferably. Maybe an extra blade)
6. Big bolt cutters
7. could you close the lock without the key? Cut away any rubber and slide a shim made from an aluminum can down to release the latch
8. Is it a cheaply made lock? Might be able to just beat on it with a hammer until the latching mechanism falls apart
9. a small car jack. Most u-locks lock on one side and connect with an L on the other, so bending the U open will release it

For the tool based approaches, I would be very tempted to work out some way with a vice or two to hold the lock and bike steady, to minimize the chances of damaging the bike.

Thieves also use leverage based attacks, but I believe those risk damaging the bike.

And, of course, having proper proof that the bicycle is yours is a good idea, too, since all those attempts to defeat a lock look suspicious.