There are two schools of thought when it comes to post production. Many photographers are quite purist preferring the idea that nature, knowledge, preparation and careful selection of a physical filter set should be enough to produce the desired image. However, a great number of photographers see post-production and digital enhancement as part and parcel of the art of photography. For general, day-to-day post production (and if you prefer not to spend too much) photo hosting sites such as Google's Picasa and Yahoo's Flickr not only provide a community with which to share your images but also offer decent photo imaging software for free.
Mid-range software includes Adobe Lightroom and Photoshop Elements. Photoshop offers some great features and filters but Lightroom is more photo-processing specific and is a worthwhile investment. With Lightroom you can add a range of filters and create pre-sets, for example re-processing your images as Black and White or creating colour separations that give your photograph a historical feel. Pre-sets in Photoshop are usually more complicated and involve programming a series of 'Actions' over various layers to get these results.
The greatest degree of flexibility is offered by the more expensive, high end software such as Adobe Photoshop CS5 but again, using an Open Source software such as GIMP is great way to get used to working in layers and adding effects and filters.
HDR processing (High Dynamic Range) is an increasingly popular method of presenting landscapes. This is a method of increasing the dynamic range between lighter and darker elements of the photograph and can be used to more accurately represent the way in which the human eye views a scene. Taken to its extreme HDR can render black and white images as ghostly and ethereal and color images as intense and highly stylised.
One of the most popular pieces of software for this is Photomatix Pro. In short, producing an HDR image involves taking 3 or more bracketed shots of the same scene using varying exposures then manipulating the differences with the software. The effect is possible with one shot but this takes a little more time and ingenuity.