There are many different types of NiMh charger so best to make sure you know what you are buying.
The most basic charger just limits the current being supplied to the cells. These are very slow and, if you inadvertantly leave them on, you can overcook the cells.
Next up we have something similar but with a timer to cut off the current after a certain time. Safer but still slow.
Next we have those chargers with some sort of charge termination. These are much faster but can still take 4 hours to completely recharge an AA cell - longer for larger capacity cells. Note that NiCd and NiMh have slightly different charge characteristics so you need to get the appropriate charger or, more commonly, one with a switch.
Then we have the fast chargers that have several charge rates and two methods (temperature and dV/dt) of charge termination. These can take as little as 15 minutes to recharge a high capacity cell. However, note that a 2500mAh cell's lifetime appears to be reduced from about 250 cycles down to 200 when exclusively charged this way.
Finally, we have the very inteligent chargers that can be configured to do whatever you want.
For one of these bells-and-whistles charge try the Maha C9000.
http://www.google.co.uk/products?q=maha+c9000
For a 15-minute charger try the Energizer
http://www.google.co.uk/products?q=energizer+15-minute
For a 30-minute charger try the U-Smart from GP batteries
http://www.google.co.uk/products?q=u-smart
Both these chargers have an external, plug top, power supply. You can get adapters to run them from a cigar lighter socket instead.
For more background information check out this forum. In paticular, look for posts by SilverFox who knows this stuff inside out:
http://www.candlepowerforums.com/vb/...isplay.php?f=9
And finally, some info about cells. The capacity of a cell is usually given in milliamp hours or mAh. The larger the number the greater the capacity. For example, a 2000mAh cell will source 200mA for 10 hours before it goes flat. These days you can get AA cells up to about 2800mAh but beware - many manufacturers lie! You really need to look at the specs instead of simply reading the number printed on the side. Also note that this increased capacity has been at the expense of longevity. Cells up to around 2000mAh seem to last for around 500 cycles but the highest capacity cells are only half that.
If you're looking for some top-notch rechargable AA cells then I'd thoroughly recommend the Eneloop from Sanyo. Although these 'only' have 2000mAh capacity they have an extremely low self discharge rate. This means that they will still work months after you have charged them - unlike ordinary NiMh cells. They will also provide a whopping output current and seem to be very reliable.
http://www.eneloop.info/