Hewlett Packard EF 50mm f/1.8 II Lens

Known as the "nifty fifty", this is amazing lens, new it's only around £60, on ebay it's even cheaper.

It's a fast lens, f/1.8, which means that it lets plenty of light in, although stepping down a stop or two really helps the picture quality.

This a tiny lens, that only weighs 130g (about 5oz in old units), and can look lost on some of the bigger DSLRs. But in low light it works far better than you would expect for it's price.

The front element doesn't rotate during focussing, so you can use graduated and polarising filers easily, but it doesn't have a bayonet for a lens hood, so you'll have to use a 52mm screw in one.

On a crop body DSLR this is effectively (equivalent field of view of) a 80mm lens, which is a little long for many indoor situations, but it does make it a good portrait lens (as it flattens the face, which gives a more flattering look).

Some people will rave about the Mk I version of this lens, that has a metal mount, and a distance scale, but the optics are virtually the same.

Summary: I don't use this lens very often, but at this price (and size and weight) I just leave it in the camera bag for when the situation suits it, and it never disappoints.