I have already built a site from scratch. It has banning, PM, comments, etc. The PMs and comments are done using markdown (like SO).

There are pros and cons for writing my own or using another software. But some cons keeping me from using another forum software is

• Multiple Logins: One for the site, one for separate forums.
• Need to Customization code: I'll need to change the toolbar in the forum software so I can access pages on the regular site.
• Look consistency: It may look drastically different from my site even after applying lots of css changes.
• Banning and User consistency. Users may be ban on site or on forums but not the other. users may select a different or multiple usernames on the forum instead of being forced to use the same username on both site and forum.

Should I write my own forum code or should I use something already written? What are some reasons for or against writing my own and using forum software?

The heuristic I tend to use is that if at least two or three of these are true, it's probably worth writing yourself:

• It's one of your core business offerings.
• You have the time, money and resource to specify, design, build, test and support it.
• The amount of time you would need to spend evaluating, prototyping with, integrating and customising off-the-shelf packages is near to or greater than the amount of time you estimate that it would take you to build your own.
• The off-the-shelf packages do not have good support. By 'good support' I mean that you can raise an issue, and be assured that somebody will look into your specific issue and provide a fix if it is found to be a bug.
• None of the off-the-shelf packages meet your quality bar for security, performance, quality, etc.
• You are not writing open-source software, and the off-the-shelf packages have a viral open-source license; be particularly wary of AGPL and other similar licenses.

One of the oft-quoted articles about this is from Joel Spolsky. Lately he seems to have just been writing flame-bait, but this was one of his worthwhile early posts.

Note that you can also take a combination approach, and use some pre-built things such as recaptcha and gravatar even from within a bespoke solution.

Writing your own forum software is nowadays in almost no case useful. The available software is highly customizable. For example, you can create your own skin to fit with the layout of your website. You can integrate the login with that of your website.

Writing your own forum is so complicated that the quality of your own software will be much lower than of something like phpBB or vBulletin. And dont forget the security bugs, the mentioned software has been reviewed by a huge amount of people. Until you reach the same quality (functionality, stability, security, etc), your website will be outdated and your forum software with it.

Once again, the current forum software is so adaptable and extensible, that it doesnt make sense to make your own.

I think it highly depends on your needs for the forum.

First off, is the forum itself an essential part of your website's function? Or is it more of a 'support' style forum?

If it's essential to your daily business, then it makes more sense to write your own - or at least highly customize an existing package.

If what you need instead, however, is a vehicle for customer support, or some sort of generic community revolving around your site, I might recommend looking at a completely different vehicle: something like http://getsatisfaction.com, or a Google Group, or some such.

(I'm in no way affiliated with either of those sites).

Regardless of the solution you choose, I'd recommend you give some serious thought to how essential the forum is. If it's not, then I wouldn't waste too much time configuring/setting one up. Instead, focus on what your site's main draw is, and spend the bulk of your time on improving that. You and your userbase will be much happier with that, IMO.