Tessar Lens

Carl Zeiss Jena Tessar 50mm f/2.8 lens (£12 on eBay) on a Canon EOS 40D digital camera.
The cheapest route to a lens with 'Carl Zeiss' on it. Manufactured between 1952 and 1978, thus there is plenty of choice on the secondhand market. The moment I had it in my hand, I loved the feel of it and the smooth focus and aperture adjustment. Using it on a crop sensor digital camera avoids the criticism that it is soft at the corners: I'm using only the best part of the simple optics. Nevertheless, optimum performance is f/8 - f/11 where contrast is best; otherwise post-processing will improve your efforts. F/2.8 is slow, but these days we're all used to levels of ISO which were unimaginable when this lens was in production, and so only photographers chasing depth-of-field will be bothered by the slowness. Close focus is about twelve inches - it's practically macro! And the long focus throw provides plenty of scope for tiny adjustments. The front element is well recessed: no need for a lens hood, nor a protective filter for that matter (but I disdain protective filters anyway. Like extended warranties, they exist to improve dealers' profit margins).
Not as good as the 55mm f/3.5 Micro-Nikkor, which is hard to beat by any lens, but much cheaper, and fun to use. Colour rendition isn't bad, either.