There appears to be many products marketed for pain relief in osteoarthritis which contain green lipped mussel, although they often list it by different names. One of the most commonly used names (other than 'green-lipped mussel or GLM) is perna canaliculus.
Green lipped mussel was first thought to be of benefit to people suffering arthritis when it was observed that the local people of New Zealand (Maoris) who regularly consumed it suffered from less arthritis than their inland relatives. Green lipped mussel extract is known to contain many omega-3 fatty acids, which in themselves are known to have anti-inflammatory activity in the body (which is why fish oil is often marketed and used for anti-inflammatory properties and pain relief).
Placebo controlled studies have found trends towards improvements in pain, as well as significant improvements in general functioning and a significant average reduction in the time it takes to walk distances. Physicians and patient opinions and assessments have also reported significant improvements with green lipped mussel treatment.
Fortunately these studies haven't reported any serious side effects to green lipped mussel treatment, however one of the reported outcomes was "increased stiffness", which is a little odd. Other side effects included flatulence, upper stomach discomfort, nausea, exacerbation of symptoms and fluid retention - however some of these side effects could be attributed to using concomitant non-steroidal anti-inflammatory medications (NSAIDs) which are known to cause many of them.
Brien S, Prescott P, Coghlan B, Bashir N, Lewith G. Systematic review of the nutritional supplement Perna Canaliculus (green-lipped mussel) in the treatment of osteoarthritis. Q J Med 2008; 101: 167-179