Paperless Post is a great site to create invitations, birthday cards, etc. There are lots of free options, and there are lots of great options to customize your card.
The Post Box lists all your invitations, including your drafts. From here, you can see who has responded to your invitation.
Once you have chosen your template, you can customize the text, the font, the background colour -- the possibilities are endless.
Here's the draft of my 50th birthday party. I had to postpone my party and details are not yet finalized.
You can preview your invitation before sending, but I love that you can send yourself a test invitation. This gives you the opportunity to see exactly what your guests will receive.
You can even customize the inside of the envelope, if you choose to include an envelope. I think the envelope is a nice touch.
I encourage you to head over to Paperless Post to take a look. There are also greeting cards, thank you cards, and stationery. It's a great way to save on printing and postage costs.