Joomla has a good RSS syndication feature. You can generate and publish RSS (and Atom) feeds easily from all of your sections and categories one by one.
What you only have to do the following: when you publish a menu item which points to a section or a category, in the block "Parameters (Advanced)", in the field "Show a Feed Link" click "Yes" and save the menu item.
If you reload your site a RSS icon and feed links (RSS, Atom) appears in the webbrowser's address bar for this menu item and your readers can subscribe to this feed.
With this method you can generate RSS feed from content items of one section or one category, and you can point it to one menu item.
Okay, it is a great feature. But what about a "main" RSS feed generated from your all content items?
Joomla has no such feature. /Or perhaps I am the only one who doesn't know this feature/
If you want to publish a "main" RSS feed on your Joomla site, you need 3th party tools, e.g. Yahoo Pipes and FeedBurner (optional).
/I need a "main" RSS feed, because I want to publish it on my frontpage and I also want to publish it on various social websites./Okay, lets do it!
Yahoo Pipes
Pipes is a powerful composition tool to aggregate, manipulate, and mashup content from around the web. With Pipes you can create your ultimate custom feed by combining many feeds into one.
Sign In to Yahoo Pipes with your Yahoo ID and look at my pipe's source http://pipes.yahoo.com/laszlo/allrssfeedsinone
I created this pipe to combine all RSS feeds of this website to one "main" feed.
Sorry, I dont want to give an overall review about all features of Pipes, but only about features I used for my pipe.
I used the fetch, sort, truncate and union modules in my pipe.
- fetch module fetches my website's RSS feeds (generated by Joomla) one by one
- truncate module limits the number of items of one fetched feed
- union module merges items of five fetched feeds into a single list of items
- sort module sorts the item elements by date descending
Again, look at into my pipe source, so you can understand better how my pipe is working. And you should know the pipe output could contain max 50 items.
Customize the pipe to your needs and save it to yourself.
Ok, you created a pipe. Your pipe output is all of your Joomla RSS feeds in one. You can get the pipe output in many formats, but we need the output as RSS. Go to your pipe's page and click the "Get as RSS" icon.
Copy the RSS link from the address bar and go to FeedBurner.
FeedBurner
/This step is optional. It is needed if you want to publish your feed through Feedburner. FeedBurner has lots of useful features, but if you don't need these features go to the next step "Joomla" and insert the RSS link of your pipe to your template./
Login to your FeedBurner account (if you haven't got it, create one) and paste the RSS link of your pipe as a new feed link / "Burn a feed right this instant. Type your blog or feed address here:"/ and click next.
Go through the next steps. /Give a name and address to your feed, etc./
Okay, you created a FeedBurner feed that is generated from all your Joomla feeds, perfect! Now go to your Joomla site.
Joomla
First, unpublish your RSS feed on your frontpage (if it has been published). /this is optional, you can hold it published/
Login to your Joomla Admin area and go to Menus>Main Menu>Home.In the block "Parameters (Advanced)", in the field "Show a Feed Link" click "No".
Save your setting. If everything is ok the RSS icon has been disappeared from the webbrowser's address bar on your frontpage.
Joomla Template
Locate the index.php file in your Joomla template.
Download the file from your server and open it in a text editor (my favourites are notepad++ in Windows and gedit in Linux).
Find the <head></head> section in the file and paste the following line after the <head> tag:
<link rel="alternate" type="application/rss+xml" title="Your Feed Title"
href="http://feeds.feedburner.com/Your Custom RSS URL Here" />
Please, insert it in one line without linebreak!!! Sorry, but there is no sufficient space in my template. 
My example:
<head>
<link rel="alternate" type="application/rss+xml" title="Laszlo Molnar -
Tech-Biz-Life-Me" href="http://feeds.feedburner.com/LaszloMolnar-Tech-Biz-
Life-Me" />
...
...
...
</head>
Okay, now save the file, upload it to your server, and reload your site.
If everything is working ok your new RSS icon appears in the address bar. If you click it, you can see your FeedBurner feed link and your content items from all your sections/categories.
Since this RSS link has been inserted to the template's index.php file, the link appears on all your Joomla pages.
On those pages where you hold the Joomla generated feed links too, all feed links will appear there, and they will be displayed in a drop-down fashion in the address bar.
That's all.
