I was ecstatic that I actually had something to comment on for once because I too got a surprise plant of the same variety from a woman whom I traded plants with.
Meet the ageratum, also called flossflower:
(closeup of leaves)Unfortunately mine isn't in bloom, I took the pics too late, but as you could see in The Home Garden blog and the following links, it has clusters of very pretty purple-blue small starry flowers
Next to a lot of certain salvias such as my pineapple sage (Salvia elegans: http://www.floridata.com/ref/S/salv_ele.cfm) and lemon balm (Melissa officinalis: http://www.floridata.com/ref/M/meli_off.cfm) when not looked too closely, the foliage can be overlooked as a match with either of those, both which happened in my herb garden.
Initially I freaked out when I saw the flowers because I had no clue as to what the heck this thing was (or if I had eaten it). Luckily when you rub the leaves of ageratum it has really no smell, especially in comparison to that of pineapple sage or lemon balm, so there was no worryin mistakening it for one of my tasty tea herbs.
When I asked about the plant to a gardener lady friend of mine, she backed away in a sort of revulsion because it is considered somewhat invasive. Therefore I yanked it out of my garden and handed if off to native plant lover... only to find later I may have rid myself of a useful plant.
Apparently ageratum has the ability to secret a chemical that affects bugs when they eat it by disrupting their juvenile hormones, rendering them infertile!
Now, while I was tempted to ask for the plant back... that's not good etiquette, so I kept my eye out alongside roads and etc, but lucky for me ageratum's supposed hardiness (invasiveness?) won the day and returned by popping out of nowhere again. I immediately dug it up and out of the herb bed and replanted it off to the side until I can find a nice permanent location for it.
I'd like to note that it's not as though all bugs like to munch on ageratum, but at least your garden has a fighting chance of rendering your bugs unable to get it on and make eggs if they do eat it and it seems that scientists are busy figuring out potential pesticides involving the chemical ageratum uses to create strife upon pests' reproductive systems.
One more final note: the ageratum I have is not the more exotic conyzoides variety, but the houstonianum sort. So, the effect may not be as lethal as the conyzoides type that is listed in science journals, but to my understanding, still effective in its own weaker fashion. Hey, whatever works right?
More pics (with the flowers) and info:
http://en.wikipedia.org/wiki/Ageratum
http://en.wikipedia.org/wiki/Ageratum_conyzoides
http://en.wikipedia.org/wiki/Ageratum_houstonianum
http://davesgarden.com/guides/pf/go/54970/
Being ex-bio major I felt that it wouldn't be correct unless I added some scientific articles for further proof:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T8Y-41XV7YW-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=989286931&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8b5b325b4b4de8db2ec15b54193ad585
http://www3.interscience.wiley.com/journal/114250531/issue
(do a word search for ageratum, doesn't let me link to the actual abstract)
