The OEM version of the Canon BP-511 is rated at 1100mAh capacity. The BP-511A is rated at 1390mAh. To the best of my knowledge Canon discontinued manufacture of the BP-511 after they updated it with the higher capacity BP-511A.

Third party vendors make BP-511 compatible batteries rated at up to 2200mAh. My experience with Sterling Tek BP-511A compatible batteries rated at 2200mAH was that they did indeed last longer than the OEM BP-511A supplied by Canon with the camera.

Some battery packs in Canon's system are interchangeable with other battery packs, but most are not. The BP-522 and BP-535, for instance, are much larger than the BP-511 and would not fit in most applications designed to use the BP-511. The BP-511 series, however, might fit in devices that use the BP-522. The lower capacity of the BP-511A means that it would not last as long compared to the 3500mAH BP-535 in those devices.

The BP-512, which was supplied with the G2 and several Canon camcorder models appears to be functionally identical to the original BP-511 (but inside a slightly slimmer case) and is also rated at 1100mAh. The BP-514 was supplied with the ZR80, ZR85, and ZR90 camcorders and appears to be the same size as the BP-512 and slightly thinner than the BP-511.

For the decision on whether to use OEM or generic batteries, see Should I buy an original manufacturer battery, or is a generic brand OK?. My answer is buried near the bottom as I didn't join this community until almost three years after the question was asked.

I've had good luck with Maximal Power and Sterling Tek batteries for my Canon cameras. Be aware that many sellers claiming to sell genuine OEM batteries are selling counterfeits, especially on eBay. Only buy OEM or generic batteries from a source you trust.