I have a leaking valve under my bathroom seak. Is there an easy way to fix it without turning off the outside water?

Yes. Is it leaking around the handle? If so, tighten slightly the bonnet nut located directly under the handle and hold the valve while doing so. Do not leave the handle all the way on unless it is a quarter turn valve.

Doug gave you a good start but there are at least two more things you can check with out shutting off the main. If it is the bonnet nut doug gave you the cure. If it is a compression nut coming off the valve going towards the sink you can tighten the compression nut. The last thing to check is how the valve (probably an angled stop cock) is attached to the supply line from the wall. If it is soldered you can fix without shutting off the main. If it is threaded you can close the stop cock and remove the line to the sink, then tighten the stop cock and replace the line to the sink. If it is a compression connection you can just tighten the nut on the supply side of the stop cock.