It looks like you've already got the Tamron 17-50 non VC, which is a great lens. For anyone else reading this who is in the same position, it might be worth considering the 50mm f/1.8 as suggested by Phil.
When I got my first DSLR (a 1000D) some 4 years ago the first additional lens I bought to complement the 18-55 kit lens was the 50mm f/1.8, and it totally changed the way I viewed photography. It barely left my camera - I was more than a little in love with the low light shooting and shallow depth of field that the wider aperture affords - and I learned to "zoom with my feet", a key skill if you're intending on picking up more prime lenses later.
Four years on and the 50mm f/1.8 is still in my kit bag, though I only use it occasionally these days. For less than £90 it represents fabulous value for money - there's a reason its colloquially known as the "nifty fifty".