To those who are not aware of it: By default the standard client and most other clients tend to link addresses to each other for example when creating payments larger than any available input. This means that all addresses that you ever used can be linked together after a while of normal usage.
If you want to keep your privacy you should be aware of this. Unfortunately there is know way of telling with most normal clients.
Blitsky has expressed it very well:
[...]
For me, coin control is mostly a privacy thing. You're always told not to reuse addresses, but if you then make a payment a bunch of different addresses are combined for the input. Everybody can see that those belong to the same wallet then. Parse the blockchain and one can group those addresses by the wallets. If one address of those can be mapped to a user, all others are automatically mapped too.
[...]
What can you do?* Use different wallets. For example with Multibit this is quite easy.
* Use Armory, it has "coin control" / sendfromaddress.
* There is a "coin control" patch for the standard client, it might be included in v0.9.
Using coin control is quite interesting from an educational point of view because it shows how addresses are connected and easily lets you understand why it happens in the first place.
coderrr's initial post:
https://bitcointalk.org/index.php?topic=24784 Patching The Bitcoin Client To Make It More Anonymous
