In playing Team Fortress 2 after the most recent update I've noticed a lot of "Vintage" items. What's different about these items? How can I acquire more?
The main difference between normal and Vintage items is that Vintage items are all the items people obtained before the Mann-conomy update. They can no longer be acquired in any way except trade. This makes them very rare. You can recognise them by their blue name only, since people could also just rename their items Vintage. Also, as pointed out in the comments, the "Vintage Tyrolean" and the "Vintage Merryweather" are renamed to "Vintage Vintage Tyrolean" and "Vintage Vintage Merryweather" respectively.
See the Mann-conomy FAQ:
Basically, Valve decided that making items available for money devalued the items, since it is less impressive to buy an item then it is to craft/find it. Therefore they converted every existing item in people's inventories to Vintage items.
Since you can no longer create them (only the items converted at the update become them) Vintage items are very rare, which gives them a higher value in trade. Otherwise they are identical.
Additionally, a couple items are excluded, such as badges, promotional items (eg. Bill's Hat) and holiday items (eg. Ghastly Gibus).
Funnily enough, the Vintage item's rarity will increase over time, as people use them up in crafting.
Vintage Items are items that existed prior to the Mann-conomy update that happened on 2010 September 30th.
You can recognize a Vintage item both by its name and name color (which is blue).
If you are trading to try and get Vintage items, note that items can now be renamed... check the name color and make sure it really is a Vintage item. Also, renamed items should have the items names in quotation marks... i.e. "Vintage Merryweather" instead of Vintage Merryweather.
There are two items that already had Vintage in their name: Vintage Merryweather and Vintage Tyrolean. These now have Vintage in their names twice for the Vintage versions.
Note: Not all items became Vintage items. Ones that didn't include:
and all the new items added during or after the Mann-conomy update (except Foster's Facade and Stockbroker Scarf, which are Vintage if you owned Killing Floor prior to the KF cross-promotion).
Unusual hats were also added to the game during the Mann-conomy update.
These are hats have particle effects attached to them, such as ghosts or a sunbeam. The names for these items are shown in purple. They are not available through the random drop system, but instead as a rare drop from a Mann Co. Supply Crate, which require you to buy keys from the Mann Co. Store (for $2.49 each) to open.
Of course, like all hats, they can be traded. Expect to trade a lot of hats for one, though, as they are exceedingly rare.
Scammers are on the prowl... if you're trading, always make sure that, while you were putting your items into the trade box, the person on the other end didn't change what items they had in it.
Strange weapons are a new type introduced with the Uber update in 2011. They are the same as Vintage and Unique items, except they also track kill counts. They have an orange name.
Edit: Dumb me, Unusual hats have a purple name, not a green name. Green names are for community and self-made items.
Vintage items are simply items that existed before the big Polycount patch. There is no functional difference between vintage items and non-vintage items. The only way that you can acquire more is to trade them from someone that already has them.
Some of the more recently added weapons post-date the Mann-conomy update but were also converted to vintage after a certain point, e.g. the Crusader's Crossbow. I wouldn't be surprised if a similar conversion was applied at some point for all the items not currently available in Vintage versions.