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Dawn of Discovery (aka Anno 1404) Gold Bundle, includes the original Dawn of Discovery game and the Dawn of Discovery Venice add-on. The bundle is currently US$9.99 at GamersGate (www.gamersgate.com).
Together, these are currently US$49.98 on Steam (not bundled and not on special). GamersGate puts the RRP of the bundle at US$29.95 (this deal is 67% off).
There are many other specials on the site but I considered this to be a stand-out.
If you're not familiar with GamersGate, they are pretty good. No client needs to be installed (a la Steam). Games can be downloaded as many times as you like; the only requirement for installation is that the computer have an internet connection so that the installation process can verify game ownership on the GamersGate server. Game patches don't seem to be applied automatically, but links to applicable patches for your games do appear in your games list.
Has some impressive reviews too