What lens to buy for my Nikon SLR?

I've just bought a Nikon D3100 and I got the standard AF-S DX 18-55 VR Lens. My question is what lens would be the perfect lens to compliment the lens I already have? I plan to shoot outdoor nature photos. I'm NOT really looking for a macro lens at this moment.

Probably a 300mm f/4 Nikkor. Not a bad lens for outdoor critter photographs.
http://www.nikonusa.com/...IF-ED.html

If you are doing landscape work you might want to study the 14mm f/2.8
http://www.nikonusa.com/...8D-ED.html

If you have any interest in zoom lenses it is hard to go wrong with the classic 70-200mm f/2.8
http://www.nikonusa.com/...VR-II.html

Sigma 150-500mm or sigma 50-500mm…

depends on your budget…

The lenses mentioned by Secular are very good but also very expensive.
The lenses mentioned by Forlorn are budget options… For values of budget that approach a grand.

For most people the definition of 'perfect lens' is based on a combination of factors which includes image quality and zoom range, but also a price bracket that matches the rest of their gear - in your case a humbly priced D3100 with the kit lens.

So in your case, I recommend the Nikon 55-200mm AF-S tele-zoom. It's the perfect companion lens for the 18-55mm, costs about $190, and is your best option for under $500. If you're willing to spend $500, get the 70-300mm AF-S instead. (Nikon also has a 70-300mm non-AFS version for just $160, but that one doesn't have vibration reduction and, more importantly, it won't auto-focus on the D3100.)