I would like to buy a 3rd party lens for my Canon 600D, I am looking more for shooting at events like parties. I'm looking for something cheap with a large zoom range.
Tamron 17-50 f/2.8 VC/Non-VC comes in mind. Both are cheap and have okay image quality. f/2.8 allows you to shoot indoor while keeping your ISO to a reasonable range. This is a good lens. The Non-VC (VC = Vibration Compensation, similar to IS) version has slightly better sharpness and contrast comparing to the VC version, but Tamrons VC is a good thing to have, I've found it better than IS in consumer grade canon lenses.
Cheap isn't good and good isn't cheap.
Good can be reasonably priced though, for events basically you never want your lowest aperture to be above f4 if you can avoid it, otherwise you'll be looking to use flash, which if you're trying to remain stealthy and go for candids will draw the attention to you.
I'd recommend looking at something like the Sigma 50-150 2.8, you can pick one up for around about £399 second hand in excellent condition. Not hugely cheap, but not hugely expensive and at 2.8 you're going to get pretty good results in not great lighting conditions, the zoom range is fairly flexible but you might want to consider having something wider for grabbing group shots as well. If you're going to be needing a wider lens then I'd say have a look at the Sigma 24-70 2.8 which you can get for around £450 second hand excellent condition.
Generally speaking, for events like parties you'd want a lens with a large aperture to enable you better shooting in low light. Unfortunately, these are usually not cheap.The ultimate solution may be Canon's 24-70 F2.8L lens - but it is awfully expensive. However, you have some alternatives:
Use an external flash. Some 3rd party flashes are ETTL compatible and can be fully utilized with your 600D.
Use a fast prime. The EF 50mm F1.8 cost around $100. It is a fast and nice lens, but it is of fixed focal length.
I am not familiar with 3rd party lenses so I have no special recommendation here.
One option is using prime lens: For example Canon 50/1.8 (not a third-party lens, but definitely inexpensive) sells for less than 100€, and in indoor conditions the f/1.8 aperture really helps you. Of course then you're fixed to one range, but even f/2.8 zooms cost 4-25 times more, and f/1.8 lets 2.4 times more light in than f/2.8, so it has some advantages.