Over the last few years, repetitive pre-recorded beats designed to have specific effects on the mind have become very popular on the Internet.
These come in several types including "binaural" and "monaural" beats. Different speeds and frequencies of these repetitive sound recordings can have different effects on the listener. For example, deep-sounding, slower beats tend to be relaxing, while faster, higher pitched beats tend to be arousing. Thus some beats are recommended for helping with issues such as insomnia and anxiety, while others are recommended for improving concentration or mental energy. Having said that, individual responses to these beats vary widely, and what one person finds relaxing another person might find irritating.
In terms of boosting concentration and focus, I've personally found the beats designed for improving IQ to be the most helpful. These seem to be moderately fast beats which relax the body, while making you feel slightly more focused and alert. A good 12 minute sample that can be downloaded for free is the Binaural "IQ increase" mp3 which is available from mentallion.com. Longer pre-recording are available for a small fee, although I haven't tried any of these out myself.
Unfortunately, there isn't much scientific evidence that these beats can provide long term benefits for specific mental disorders like ADHD. But given that you can trial them for free (or for a small fee) and there are no reported side effects, I'd recommend giving them a go.