Forget the envelopes, print your logo/URL etc on some labels.
Business Cards - yes, be creative (possibly a different shape or thickness)
Invoices - yes, you can create these from your accounting software or Excel (with your logo and URL)
Brochure - seems to be popular. Again, as someone mentioned it may depend on what business you are in.
Maybe a decal (with company name/URL) for your car?
The key is to be original or you stuff will get lost in the sea of printed materials. Start small (business cards and maybe a brochure) and see how your customers react.
As well, snoop around and see what your competitors are doing.
you might want to check out:
http://www.gmarketing.com/
I personally think alot of companies waste a ton of money on printed materials. Many successful companies I know of just have the cards with everything else printed on demand (invoices, quotes, etc).