I love my dremel! takes probably a little longer than angle grinder but makes a nice cut. Another plus is you can get all the way to the edge and make the hole bigger. Cost me about $10 in discs to do a top and about 30 mins.
I bought some discs and borrowed a friend's angle grinder. Cut through like butter. Well, maybe not butter, but it was pretty easy. However you do it, where hearing protection and goggles.
I have a air powered cutoff wheel it worked awesome for cutting a very nice hole in the top of my keggle but if I had a choice I'd use a plasma cutter.
if you're like me and lack any of the above tools, do what i did. i took it to a welder and he cut the tops off and put spigot holes in two kegs for $25. hardly worth the hassle of doing it yourself at that price.
$18.99 for an angle grinder and $3.99 for a step drill bit at Harbor Freight, and 20 min in the garage, and you have a keggle. I was so easy, and I have used the grinder a ton since.