First up we have The Pretty Blog... they have tons of great resources, especially for weddings, and under their "Do-Able DIY's" section they have tons of great printables, like Save the Dates, Valentine's cards & ideas, party stationery, and more.
Another great resource is over at Everything Etsy, where they have *gasp* a list of 101 Pretty Printables that are free, and their sources! A few of the printables from Everything Etsy come from this slideshow on HGTV.com with the most adorable invitation printables.
Next is from the Ruffled blog, which also has tons of great wedding inspiration, and some freebies to download as well!
And lastly, but definitely not the least! - is the Wedding Chicks. What's especially awesome about their offerings is that the website customizes it for you, with the color you choose and information / names you input. They have customizable monograms, invitations, and save the dates.