As has been said many times, a weed is just a plant in the wrong place. My garden is full of them at the moment, growing from gravel and cracks in paving stones and from pots, but I usually leave them for a while to identify them before having a bit of a tidy-up at the end of summer.
Here are some of my recent pictures. If these strays were 10 times bigger we might think better of them and make them part of our planting schemes…

Wood avens (Geum urbanum) is so hard to get rid off - the rosettes of leaves break off at ground level rather than allowing you to pull out the root. And letting them go to seed like this doesn't help...
(ADDED LATER: Eventually I think I did identify it. See here)

... until it turned into this, which is clearly a member of the Cruciferae family, maybe even a brassica... then I realised (LOL) - I had planted some French breakfast radishes and they had gone to seed!
POSTSCRIPT:
In a comment elsewhere Paul has kindly identified the little willowherb for me – here’s his picture of Epilobium obscurum for comparison. The common name is “short-fruited willowherb” but in W Keble Martin’s Concise British Flora in Colour it’s also called “thin-runner willowherb”…
I read this at the right moment, having just posted a similar (though less elegant) eulogy of (to?? . . . uncertain of my grammar here!) ‘weeds’.
http://looseandleafy.blogspot.com/2011/07/ones-that-got-away.html
Lucy
Loved your post – just Tweeted it.
Have a great trip and see you in September – I look forward to the pictures!
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[…] Identifying WeedsDo you regularly control weeds? Some gardeners let them grow for a while to identify them: http://bit.ly/qhmfGw […]
Some weeds looks like they’re part of the garden. đŸ™‚ Some are even prettier than the flower. đŸ™‚ I sometimes consider keeping the plant-like weeds instead of pulling them out. They add some beauty in my garden. đŸ™‚