The Sigma AF 180mm f/3.5 EX DG HSM APO macro is a modern autofocus lens which was produced for many mounts, but is sadly no longer in production. Mine is the Canon version. Compared to Canon's 180mm f/3.5, this lens is sharper and cheaper.
My copy of this lens has a little fungus inside, but it doesn't have a noticeable impact on the photos, which are sharp and contrasty. The combination of close focus distance, long focal length, and fairly fast aperture allows the background to be completely blurred, which makes this lens an excellent choice for floral macro. For insect macro, the extended working distance afforded by the 180mm focal length allows you to stay further away from skittish insects and still achieve 1:1 magnification.
This lens can be combined with a stack of extension tubes to provide a modest boost to its maximum magnification. However, the best option for increasing magnification is probably using a supplementary close-up lens, or by adding a teleconverter.
This is the first lens I've shot with that has a tripod mount, and I've found it surprisingly useful. It allows you to switch between landscape and portrait orientation by simply rotating the lens in its ring, without shifting your tripod head or the framing of the central part of your image.
The hood for this lens is deep and effective. Near 1:1 magnification or higher (with tubes) you will want to remove the hood to avoid spooking insects or casting a shadow on your subject. Though when reversing the hood onto the lens for storage, it makes the focus ring completely inaccessible, which is annoying. You'll want to put it away in a gear bag instead.
Optical limits has a review for this lens which includes resolution test results.