By Alfred Hermida BBC News Online technology staff

It has been described as the Swiss army knife of handheld communicators, letting you make phone calls, send e-mails and instant messages, manage your appointments and contacts and play games.

The Sidekick developed by Californian start-up Danger is one of a new brand of gadgets, which aim to bring together mobile phones and handheld computers.

Since it was launched in the US in October, the wireless Personal Digital Assistant (PDA) has sold tens of thousands.

"We exceeded our goals with little or no publicity," said Danger Chief Executive Officer Hank Nothhaft.

"There was a natural buzz around the Sidekick," he told BBC News Online.

Web to go

Fans of the PDA in Europe can expect to see it in the shops by mid-summer.

Nothhaft: "Truly convergent device"

The model on sale in the US has a 2.25-by-1.5-inch monochrome display, which rotates open to reveal a small but usable keyboard.

As well as being a mobile phone, the Sidekick offers an always-on connection to the internet via a fast GPRS link.

It can be used to surf the internet and send e-mails and instant messages.

Danger has developed software that compresses web pages to about 10% of their existing size.

"We aim to provide high quality performance without eating up the bandwidth," said Mr Nothhaft.

Mobile investment

Danger is currently in talks with various big brand manufacturers to develop their product.

It sees itself as software rather than a hardware company, earning its money from licensing the programs and hardware design behind the Sidekick.

The Sidekick device started life a year ago as a prototype designed to tempt investors.

Since then both T-Mobile and Orange have invested in Danger, which was started by engineers from Apple and another tech firm called General Magic.

T-Mobile released the Sidekick in the US three months ago, with price plans ranging from $39 to $59 a month.

The cost of the device itself can vary from $50 to $250.

Teen spirit

While no exact figures on sales are available, Danger says they are in the tens of thousands.

We need to move as if tomorrow was our last day on Earth Hank Nothhaft, Danger CEO

Instead of a big ad campaign, the company has relied on word of mouth and rave reviews in the technical media.

The Sidekick is aimed at a young audience and around two-thirds of the people who have bought it are aged 18-24 years old.

Of its customers, half use the web every day and send and receive 50 messages a day.

Fast mover

Danger is improving the Sidekick ahead of its launch in Europe in the summer.

It is planning a colour screen and improved web browser, as well redoing the way the phone works after several critiques.

In the long term, Danger aims to license its technology rather than build the devices itself.

"We want to be an industry-shaping company, so we need to partner in next 12-18 months," explained said Mr Nothhaft.

"We need to move as if tomorrow was our last day on Earth."