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.
I know you asked for a third-party lens, but maybe the kit 18-55 lens from Canon (EF-S 18-55mm f/3.5-5.6 IS II) is what you want?
- at 18mm it's a respectable f/3.5, less than a stop slower than the f/2.8 lenses others have recommended.
- there are very few lenses that are cheaper than this lens (assuming you don't already have it if it came with your camera).
- 3x zoom is a lot better than it sounds, and with 18 megapixels you could just crop photos of subjects that are farther away and probably still get a good image out of it.
- your 600D should have pretty decent high ISO performance, so if it's dark you could shoot at ISO 1600, or even higher if necessary, to maintain a fast shutter speed and probably end up with good pictures.
- if it's really dark, a more-expensive lens won't help as much as using your flash; if you're using your flash, you don't need a more-expensive faster lens to freeze the action.
Finally: in my opinion, the best way to decide what lens you should buy is to use any lens for a long-enough time (let's say around 1,000 pictures). Once you've taken enough pictures, you should have an idea of what your lens can't do for you (Depth of field not shallow enough? Doesn't zoom in far enough? Doesn't zoom out far enough? Focuses too slowly? Despite good technique, photos aren't sharp enough?) and you can zero in on the lens you need to fix the specific problem you are having (and if you have more than one problem, you'll probably need a different lens to solve each problem). And this lens (or whatever lens you already have) will help you do that.
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.