There currently is no auction. When an auction is created, it will be linked at the top of this page.

Forum ad slots are auctioned off periodically. Here is the current auction. Here are the ad stats. See the current auction thread for more info.

In addition to the inhouse forum ads, it is common for forum users to sell ad space in their signatures. These ads are not intermediated by bitcointalk.org in any way. If you are interested in advertising in this way, it is recommended that you hire a signature manager.

Main slots (1 in 10 chance)

Nine are auctioned off. One is the factoid slot.

Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Once a transaction has 6 confirmations, it is extremely unlikely that an attacker without at least 50% of the network's computation power would be able to reverse it.
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.
Factoid (see below)
Advertised sites are not endorsed by the Bitcoin Forum. They may be unsafe, untrustworthy, or illegal in your jurisdiction.

Factoid slots (1 in 570 chance)

The tenth slot is usually reserved for these factoids. Occasionally the tenth slot is instead temporarily sold (at a higher-than-normal rate) when somebody is desperate to buy it; in these cases, there is temporarily no chance of a factoid appearing.

Factoids fall into two categories: First, anyone can request that a factoid be added (for free) if it would be helpful to the community. The factoid can be a short bit of info or a simple, mostly-unstyled link. Links to for-profit sites/projects are much less likely to be accepted. Create a topic in Meta if you would like to request a factoid, and the community will debate whether it is actually sufficiently useful to be added. Second, anyone who can see the private Donators section is allowed to add one factoid with few restrictions; this type of factoid explains the existence of the few clearly-for-profit factoids.