I'm no expert on sound, but for Internet productions, I use the free ambience plug in. Works for me, start with a preset then tweak away to your hearts content. Price is right. If interested, just google:'ambience VST'
Duncan

Acoustic Mirror that ships with Sound Forge lets you take impulse recordings from a location and then apply them to a dry recording.

A copy of Audio Postproduction for Digital Video by Jay Rose although a book is not a bad investment for information about audio for film. It's quite old now but the principles remain the same.

Bob.

I'll second Acoustic Mirror from SF, but may be a bit heavy CPU-wise to use multiple instances.

You can download impulses from many famous or generic real places, or make your own.

http://www.sonycreativesoftware.com/download/impulses

geoff

You already have my favorite all-purpose ambient plugin, which is Trueverb, The interface is hard to get used to, but it's very powerful for creating a space where one doesn't exist. You don't say what type of environment you're wanting to create, or if you're trying to match an existing ambience. If this is for ADR, or matching close mics to distant mics in the same shot, you can't beat Acoustic Mirror, or another convolution reverb, You can almost always find something in the location track you can create an impulse file from. AM with existing impulse files will always give you a great sounding result, but I generally will go to Trueverb if I just want to quickly enhance the room sound.

I'll add my recommendation of Acoustic Mirror. If you need to simulate different acoustic environments it works much better than a simple reverb plugin.

AM is great. If you want to get real picky, you can create your own impulses when you're on location.

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I love Altiverb.